With plpbt.img in a (virtual) floppy drive as a second-level multi-boot, VirtualBox is an EXTREMELY useful tool.
It can access or boot a CD/DVD (real or virtual ISO) such as SysRescueCD, any system on a virtual disk in a host file, and NOW, OWING TO PLOPBOOT, also on a USB drive.
So, with a single computer running a Linux system such as Ubuntu, any disk drive can be put in an external case to be rescued with the many tools that can be started that way, including Windows.
Best of all, a complex multi-system disk can be built and tested on an external disk without any disk or CD swapping except for a Plug & Play swap of that disk with the disk inside the computer that built it. You can even take screenshots of the installation without buying a camera!!!
PLOPBOOT is much safer that the VBox disk mapping method which can mount a partition twice and destroy it.
Hence, a solution for this problem is EXTREMELY WELCOME too.
The full version of VirtualBox is free and very easy to use for testing this issue.
THANK YOU.